Recently, a fuel cell attracts much attention, expected and intensively developed as one of measures of resolving an energy or environmental problem. Particularly, a fuel cell using a solid polymer film currently constitutes a main stream of developing a fuel cell since an operating temperature thereof is as low as about 80° C. to be handled easily. However, since the fuel cell uses hydrogen as a fuel, a hydrogen detecting gas sensor is needed as a safety measure against leakage thereof.
In a background art, there is adopted such a gas sensor having a principle of utilizing the fact that a thermal conductivity of hydrogen is extremely larger than that of other gas and detecting a change in a thermal conductivity by presence of hydrogen as a change in a temperature of a heat generating element. According to the gas sensor, when hydrogen is present in air, a heat amount deprived from the heat generating element is larger than that in the case of air, thereby, the temperature of the heat generating element is changed in accordance with a hydrogen concentration. The temperature change is electrically detected as a change in a resistance value of the temperature detecting element.
The heat generating element used in the gas sensor is used also as the temperature detecting element, and as a material, a part thereof a thin film resistor made mainly of platinum is used. Since the thin film resistor is constituted by a thin film, the thin film resistor can be fabricated by utilizing a semiconductor micromachining technology, that is, a micromachining technology. Thereby, an extremely small heat generating element can be formed to provide a characteristic of capable of achieving high speed formation of detection and response of the gas sensor and achieving low power consumption formation.
As reference information with regard to the invention of the application, for example, Japanese Patent Unexamined Publication No. 8-101156 (reference 1) is known.
When the gas sensor is used for detecting leakage of hydrogen, presence of moisture in the detected gas (hydrogen) poses a problem. That is, when moisture is not present, the resistance value of the heat generating element is changed in accordance with the hydrogen concentration. When moisture is present, the resistance value is changed also thereby, and it is difficult to differentiate whether the change is caused by hydrogen or moisture, or a synergistic operation of both.
In contrast thereto, the above-described background art utilizes the fact that by making a current supplied to the heat generating element comprising the platinum thin film resistor variable, an output voltage is changed in accordance with a degree of reaction of the heat generating element. By substituting voltages across both ends of the heat generating element when supplied with the current for a previously calculated estimation equation and simultaneously calculating the voltages, an amount of the atmospheric gas, that is, concentrations of respective gases are calculated from a solution of the estimation equation.
Basically, gas concentrations of a plurality of components can be calculated by such a solving method. However, what poses a problem is a case of detecting a state of leaking hydrogen into the atmosphere in a state in which steam near to 80° C. is substantially saturated as in detecting leakage of hydrogen of a fuel cell. In such a case, when changes in the thermal conductivities of the respective gas components are represented by linear equations as in the background art or fall in ranges regarded to be linear equations, the changes can be calculated by using Chevyshev's orthogonal polynomial.
However, in a system in which steam is conceived to be present by an amount larger than that of hydrogen, a nonlinear property of the mixed system in which the thermal conductivity temporarily rises with moisture and drops after a peak necessarily shows a property having a secondary order or higher degree and therefore, the calculation becomes complicated by simply solving the changes by simultaneously calculating the estimation equation. A plurality of pieces of solutions with regard to moisture are present and the moisture cannot uniquely be determined and therefore, there is brought about a drawback that also the hydrogen concentration cannot uniquely be determined.
In order to overcome such a drawback, the inventors have already proposed the technical thought with regard to a moisture correcting system capable of correcting moisture by utilizing the fact that a heat generating temperature of the heat generating element is made to differ and a difference of outputs of the heat generating element at respective temperatures correspond to moisture by a one-to-one relationship (Japanese Patent Unexamined Publication No. 2004-354210). When a gas sensor is operated by a system of switching a plurality of constant current sources based on the moisture correcting system as described in the reference 1, a detection accuracy of the hydrogen concentration is ±0.5% H2 (% H2 designates hydrogen concentration. same as follows), and the system cannot be regarded necessarily as excellent as a method of detecting the hydrogen concentration of percentage order as in detecting leakage of hydrogen.
The reason is that by correcting moisture, a hydrogen sensitivity becomes small by about one order or more, and an S/N ratio is deteriorated. Therefore, according to the system of switching the constant current sources as described in reference 1, the S/N ratio is increased and the drawback still remains.